Bacterial vaginosis (BV) is a vaginal condition that develops when the concentration of healthy Lactobacillus species in the vagina declines and is replaced by other (largely anaerobic) bacterial species. BV is the most common vaginal infection worldwide in women of reproductive age. Pregnant women with BV have increased risks of spontaneous abortion, preterm labor, preterm birth, chorioamnionitis and other detrimental obstetric and gynecologic outcomes. According to a recent meta-analysis of more than 30,000 women, prevalent BV was associated with a 60% increased risk of acquisition of human immunodeficiency virus (HIV). However, the etiology of this complex condition is not clear. We hypothesize that low vitamin D levels contribute to BV pathogenesis. Vitamin D is essential to immune function, serving both to stimulate mechanisms associated with pathogen elimination and to regulate immune response. In the United States (US), the strongest risk factor for BV is non-white race, and non-white women are also much more likely to have insufficient or deficient vitamin D levels. Indeed, two recent cross-sectional analyses report that vitamin D insufficiency is associated with prevalent BV in pregnant African-American women; a third publication found the same association between vitamin D and BV in a nationally-representative population of pregnant women including all racial/ethnic categorizations. These data suggest that the consistent association between race and BV (despite adjustment for other BV risk factors) may be mediated by vitamin D. Little data exist on vitamin D levels in resource-limited settings, including sub-Saharan Africa where BV prevalence is high and BV- associated morbidities are substantial. Using stored serum samples from nearly 600 Zimbabwean women who previously participated in the prospective, observational Hormonal Contraception and Risk of HIV Acquisition (HC-HIV) study, we propose to measure the association between serum vitamin D levels and a) BV prevalence; b) BV incidence; and c) BV persistence. In an exploratory analysis using stored cervical and serum samples from a subgroup of 50 BV-positive and 50 BV-negative women, we will also evaluate correlations between BV-associated cervical immunoinflammatory mediators and vitamin D-associated serum immunoinflammatory mediators to elucidate the possible immunological mechanisms through which vitamin D may affect BV. Vitamin D is safe, inexpensive and has many health benefits. If vitamin D is also associated with BV, supplementation with this essential vitamin may have substantial impact on women's reproductive health worldwide.